1. Field of the Invention
The present invention relates to an internal combustion engine with a dual turbocharger system which can effectively suppress a surging which may occur when a throttle valve is rapidly closed without causing melting of a compressor impeller.
2. Description of the Prior Art
In a turbocharged engine, when a throttle valve is rapidly closed, the pressure upstream of the throttle valve rises due to the inertia of the intake air and the rotational inertia of the compressor. The pressurized air sometimes generates pressure reflection at the throttle valve and flows upstream toward the compressor to generate a surging and a sound at the compressor.
Japanese Utility Model Publication SHO 63-983 proposes to provide an air bypass conduit bypassing the turbocharger compressor and an air bypass valve installed in the air bypass conduit. The air bypass conduit and the air bypass valve return the pressurized air between the throttle valve and the turbocharger compressor back upstream of the compressor by momentarily opening the air bypass valve during a rapid deceleration to suppress the occurrence of a surging. The air bypass valve includes a diaphragm, a first chamber on one side of the diaphragm connected directly downstream of the throttle valve, and a second chamber on another side of the diaphragm connected downstream of the the throttle valve via a pressure transmittance delay valve. Accordingly, the air bypass valve operates due to the pressure difference generated between the first and second diaphragm chambers.
However, circulation of the intake air through the air bypass conduit and repeated compression of the intake air by the compressor will increase the temperature of the circulated air, which in turn may melt the compressor impeller. In addition, because of the time delay necessary for the intake pressure downstream of the throttle valve to be transmitted to the diaphragm chambers, the air bypass valve responds slowly, further causing the temperature of the compressed air to rise. Furthermore, provision of the air bypass valve alone might be insufficient in some cases for suppressing a surging during a rapid deceleration.